The past decade has witnessed increased public and industry awareness of the need to utilize all available energy resources with maximum efficiency. One area of particular interest is the utilization of so-called "waste heat" that is associated with many, if not most, heavy industrial processes. The recovery and utilization of such heat provides potential benefits in terms of increased efficiency of production in the food processing, petroleum and refining and energy production industries, for example.
In all of the named industries, thermal energy constitutes a major process by-product. Limitations upon the attainable efficiency of energy utilization necessarily result in the loss of some thermal input via nonproductive radiation and the like. Numerous heat exchangers have been devised for transferring the heat stored in a first medium to a second medium for subsequent use or disposal. However, various drawbacks have limited the efficiency and versatility of heat exchangers in handling a wide range of fluids, especially those of high temperature and high pressure. Several features are essential for efficient heat transfer in shell and tube type heat exchangers. Frequently, multi-walled tubes are employed where two fluids must be protected against mixing even when a leak occurs.
A large tube surface area is necessary for effective heat transfer and the surface area increases with tube length and tube diameter. However, the advantage gained from a larger tube diameter is offset by a decreased heat exchange which results from a fluid inside of the large tubes tending to flow through the middle area of the tube where heat transfer is lowest rather than adjacent the peripheral tube wall where heat exchange is greatest. A long tube length poses a problem with longitudinal expansion. When a high temperature shell fluid is processed, the tube temperature increases and the multi-wall tubes expand individually. To avoid overstressing any of the multi-wall tubes, expansion means are needed for each individual tube. In addition, the design of the tube expansion means should be able to accommodate a leakage detection system.
Another factor affecting the rate of heat exchange is the flow of the fluids in relation to each other. Optimum heat transfer is achieved when the shell fluid and tube fluid are in contraflow relation on every pass. To achieve the multipass but contraflow relationship, leakproof baffles are needed to keep the shell fluid passing sequentially through one chamber at a time and to reverse the direction of flow at each end of the shell. Headers are needed to divert the tube fluid flow through the tubes into several sequential passes through the heat exchanger.
The use of a long multi-pass heat exchanger handling high temperature, high pressure fluids is not practical since the thermal stress at each end that is induced by temperature differentials causes the whole heat exchanger to bend or beam, resulting in intolerable mechanical distortion. High pressure, high temperature fluids that are reactive or corrosive to tube material present other problems in heat exchanger design. It is important not only to keep these fluids isolated from each other to prevent contamination but also to provide an efficient means for leak detection for the multi-wall tubes. To facilitate low cost maintenance it is essential to provide for quick access to the internal heat exchanger elements so that such elements can be readily interchanged with minimal time and effort.
U.S. Pat. No. 1,683,236 to Braun discloses an integral shell and tube heat exchanger with multi-pass tube flow but only double pass shell flow directed by a single central divider. This design decreases the heat transfer efficiency because it can not achieve complete counterflow on each pass of shell fluid in relation to tube fluid. The use of single wall tubes secured in fixed tube plates prevents efficient leak detection between the transfer fluids and does not allow for tube expansion. The integral shell provides no means to reduce the thermally induced stress that causes mechanical distortion in high temperature multi-pass shell and tube heat exchangers.
U.S. Pat. No. 1,790,828 to McKnight discloses a four-pass shell and tube heat exchanger with contraflow on each pass of the shell fluid in relation to each pass of the tube fluid. This four pass contraflow is achieved through a longitudinal, vertical and horizontal baffle that extended through the shell, dividing the shell into a plurality of water tight chambers. This design is satisfactory for pre-heaters where temperatures are low but not for applications requiring higher temperature refrigerants. This design fails to provide for tube expansion that occurs at higher temperatures and does not take into account thermally induced mechanical distortion that occurs in high temperature shell and tube heat exchangers of the multi-pass type.
U.S. Pat. No. 1,672,650 to Lonsdale discloses a shell and tube heat exchanger with a floating head and multiple baffles welded to an inner central tube. The ends of the baffles are fitted into resiliently packed slotted tubes which are in turn welded to the shell. Although this design achieves multi-pass shell flow, it only allows for double-pass tube flow which results in inefficient heat transfer since complete counterflow is not obtainable. The baffle and tubes can be taken out of the shell for repair and replacement, but substantial effort is required to maintain the leak-proof joint in the slotted tube which is sealed with packing.
German patent No. 2,111,387 discloses a horizontal shell and tube heat exchanger. Single wall tubes are used with neither leak detection nor expansion means. The tubes are fixedly attached at each axial tube end to a tube plate in each end cover. A liquid or gaseous medium could be used as a shell fluid and radial and longitudinal baffles extend the entire length of the shell to provide for a multi-pass shell flow. Radially extending partitions in the deflecting end covers provide a multi-pass flow through the tubes. The longitudinal baffles are attached to a central tube which extends from one tube end plate to the other. This connecting tube increases the stability of the baffles. Although this heat exchanger could be operated in co-current flow or countercurrent flow, the heat exchanger's application is limited by temperature and pressure restraints.
Another problem present in the art of shell and tube heat exchanger is an accurate, efficient method for the leakage detection between transfer fluids.
U.S. Pat. No. 1,738,455 to Smith discloses a steam condenser that utilizes a double walled tube. A high pressure fluid flows within an outer tube wall which surrounds an inner tube wall containing a lower pressure contaminating fluid. If the inner tube leaks, the difference in pressure between the two fluids prevents fluid in the inner tube from leaking out and instead forces the higher pressure fluid to leak into the inner tube. This double wall system effectively isolates the contaminating fluid when the inner tube leaks but does not provide a leakage detection system for a heat exchanger.
In the Smith system each tube end of the double wall tube configuration terminates in a separate tube plate sealed by packing. In order for the Smith arrangement to achieve readily accessible and replaceable tubes without removing the outside tube plates, the tube plates are provided with openings that are sufficiently large for the outer tubes to pass through them. A disadvantage of this system is that the tube ends terminating inside each tube plate must be excessively packed to prevent leakage and the inner tube must have its tube end expanded to compensate for the tube plate modification for the outer tube.
British patent No. 273,605 to Thornycroft discloses a steam condenser wherein the steam is condensed by a passage of cooling water through single wall tubes that extends between two tube plates in each end of the condenser. The tube ends are not fixedly attached to the tube plates and are free to longitudinally expand. Each tube end is packed with packing rings which are compressed in place by ferrules. The ferrules screw into each tube plate to form a watertight joint. Surrounding the tube ends between the two tube plates is a fresh water chamber. As in Smith's patent 1,738,455, if the tube end leaks, seawater inside the tubes, being at a lower pressure than the fresh water outside the tubes, cannot leak out and contaminate the fresh water.
Other practical considerations in any design of a shell and tube heat exchanger include the accessibility and replaceability of the internal elements and means for compensating the internal elements in accordance with temperature changes.
U.S. Pat. No. 730,284 to Pepper discloses a double wall system for a shell and tube heat exchanger that utilizes a vent chamber and bonded tubes. The tube ends are fixedly attached to two tube plates at each end of the shell with a space therebetween. The outside surface of the inner tube wall has helical channels or grooves cut into it so that leaking fluid can flow along the tube length to a vent chamber for detection. The disadvantage of bonding is that it prevents any longitudinal expansion of the tubes and the tube ends must be fixedly attached to the tube plates to seal the vent chamber for efficient leak detection. Bonding also increases tube cost and fixedly attached tube ends prevent use of accessible and replaceable tubes.
U.S. Pat. No. 2,658,728 to Evans discloses a method for longitudinal expansion of double wall tubes by having expansion joints on the shell. Evans uses two bellows type expansion joints. A first joint compensates for expansion of the outer tube and a second joint compensates for the expansion of the inner tube. These expansion joints increase the cost of shell construction and require both tube ends to be welded in respective tube plates. This construction eliminates efficient accessibility and replaceability of double wall tubes. Nor does this arrangement accommodate differential expansion of different chambers within the heat exchangers.
British patent No. 619,585 to Newling discloses a vertical shell and tube heat exchanger with a lining between the shell and tube bundle to reduce the amount of transfer fluid that flows through the tube area. The shell must be constructed with a large bore that enables the tube bundle and the floating head to be removed as a single unit in the event of repair or replacement. The shell fluid inlet and outlet ports are not sealed between the shell and the lining so that a thick axially extending space contains a thick layer of stagnant shell fluid. Although this thick layer of stagnant fluid acts as a thermal insulator it does not reduce the thermally induced stress that causes mechanical distortion in high temperature multi-pass shell and tube heat exchangers.
U.S. Pat. No. 3,768,554 to Stahl discloses a vertical liquid-metal vapor generator with a wrapper sheet separating a tube bundle from the generator's shell. An annular space between the wrapper sheet and the shell shields the shell from rapid temperature transients. A layer of liquid metal six inches thick fills this annular space and remains stagnant throughout the generator's operation. This type of shielding utilizes the thermal conduction resistance and heat capacity of the liquid metal itself to decrease the heat transmission.
U.S. Pat. No. 4,114,598 to Van Leeuwen discloses a solar heater with two sided extrusions interlocked in a tongue and groove fashion. This method of interlocking is practical for solar heater elements lying in a horizontal plane but would be of no use in forming the circumferential shell of a heat exchanger. A circumferential pressure vessel shell needs three line locks to sealingly interlock the shell and a radial and circumferential segment on the extrusion to form the shell and its inner chambers.
U.S. Pat. No. 825,905 to Hellyer discloses a drying machine where a series of triangle cells mounted and interposed between the walls of a jacket surround a cylindrical main body portion. Although the cells are removable and form a symmetric outer shell, they have no interlocking elements or radial and circumferential segments that form a segmented and baffled self sealing shell for use in a heat exchanger of the shell and tube type.
In summary, while a shell and tube type heat exchanger presents a relatively simple design, there are a number of problems that have reduced its overall efficiency in its present state. There exists a need in the art for a shell and tube heat exchanger of low cost modular construction that is easy to maintain and repair and that can meet pressure vessel regulations while yielding high efficiency heat transfer over a wide range of fluids. A high quality, multi-pass heat exchanger should provide an efficient leak detection system that allows for individual longitudinal expansion of multi-walled tubes and a means to substantially reduce thermally induced stress that causes mechanical distortion while keeping the cost low.